The field of the invention is gyromagnetic resonance spectroscopy, and particularly, local coils which are used in such systems.
Gyromagnetic resonance spectroscopy is conducted to study nuclei that have magnetic moments and electrons which are in a paramagnetic state. The former is referred to in the art as nuclear magnetic resonance (NMR), and the latter is referred to as paramagnetic resonance (EPR) or electron spin resonance (ESR). There are other forms of gyromagnetic spectroscopy that are practiced less frequently, but are also included in the field of this invention.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z) the individual magnetic moments of the paramagnetic nuclei in the tissue attempt to align with this field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented components in the perpendicular plane (x-y plane) cancel one another. If, however, the substance, or tissue, is irradiated with a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, can be rotated into the x-y plane to produce a net transverse magnetic moment M.sub.1 which is rotating in the x-y plane at the Larmor frequency. The degree to which the rotation of M.sub.z into an M.sub.1 component is achieved, and hence, the magnitude and the direction of the net magnetic moment (M=M.sub.0 +M.sub.1) depends primarily on the length of time of the applied excitation field B.sub.1.
The practical value of this gyromagnetic phenomena resides in the radio signal which is emitted after the excitation signal B.sub.1 is terminated. When the excitation signal is removed, an oscillating sine wave referred to as an NMR signal is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.1. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of M.sub.1.
The NMR systems which implement these techniques are constructed in a variety of sizes. Small, specially designed machines, are employed to examine laboratory samples or animals, or to examine specific parts of the human body. On the other hand, "whole body" NMR scanners are sufficiently large to receive an entire human body and examine any portion thereof. The NMR signals received may be used to identify the presence and relative concentration of substances within the field of view by the characteristic NMR frequencies, or "signatures" of known atomic structures. By scanning many points within the field of view an entire image can be constructed, which is particularly useful for anatomical studies.
Whole body scanners may employ a separate excitation coil and local coil for transmitting the excitation field and receiving the NMR signal, respectively. The excitation coil produces a highly uniform, or homogeneous, excitation field throughout the area of interest, which is a principal advantage of a whole body scanner. The local coil is placed near the area of interest to receive the NMR signal. The local coil consists of a resonator which is sharply tuned to the Larmor frequency of the nuclei of interest.
Recently a novel resonator structure, referred to in the art as a "loop-gap" resonator, has been applied to the field of gyromagnetic resonance spectroscopy. As indicated in U.S. Pat. Nos. 4,435,680; 4,446,429, 4,480,239 and 4,504,788, the loop-gap resonator may take a wide variety of shapes. In all cases, however, a lumped circuit resonator is formed in which a conductive loop is the inductive element and one or more gaps are formed in this loop to form a capacitive element. While the loop-gap resonator has many desirable characteristics normally associated with lumped circuit resonators, it also has some characteristics normally associated with cavity resonators. Most notable of these is the much higher quality factor, or "Q", of the loop-gap resonator over the traditional lumped circuit resonators.
A major technical problem in NMR systems is decoupling the local coil during the excitation portion of the measurement cycle. Decoupling is necessary to prevent distortion of the excitation field and to prevent potential damage to receiver input circuits by large voltages from the local coil. This problem is compounded by the very high Q of the local coils, especially those employing loop-gap resonators.